1. Field of the Invention
The present invention relates to a class of high isolation broadband amplifiers and more particularly to increasing the efficiency of a broadband amplifier without adversely affecting its bandwidth or reverse isolation characteristics.
2. Discussion of the Prior Art
In the design of broadband amplifiers, feedback is typically used to increase bandwidth and optimize high frequency response. However, simple shunt feedback causes the amplifier to have low isolation (the suppression of a signal applied at the output port as measured at the input port). This can cause unwanted parasitic cross-coupling between the input and output circuits.
One way of providing feedback while maintaining high isolation is to use ferrite directional couplers, as disclosed in U.S. Pat. No. 4,042,887. See FIG. 1. However, the bandwidth of the amplifier is limited by the bandwidth of the ferrite couplers. Also, the ferrite couplers are relatively expensive.
A broadband high isolation amplifier which avoids ferrite couplers is shown in FIG. 2, and its high frequency equivalent is shown in FIG. 3. Transistors Q.sub.3 and Q.sub.4, and resistors R.sub.1 through R.sub.5 constitute the bias circuit. All capacitors are bypass (blocking) capacitors. Resistor R.sub.6 sets the output impedance at 50 ohms to match the load. The collector current of Q.sub.2 is split between R.sub.6 and the load, and it is proportional to and in phase with the load current. Ignoring base currents, the current through R.sub.7 is therefore proportional to the load current, and the voltage across it is proportional to the output voltage. This voltage across R.sub.7 is used to apply feedback to the base of Q.sub.1. The high isolation of this amplifier is realized by the inherent isolation of Q.sub.2 in the common base configuration and by not sampling the feedback voltage from across the load.
Since R.sub.6 is in parallel with the load, only half of Q.sub.2 's collector current is available for the load, making the amplifier perform with only moderate efficiency. The prior art technique for improving the efficiency of this circuit is shown in FIG. 4. A transformer T.sub.1 with a four to one turn ratio is placed across R.sub.6, whose resistance is increased to 200 ohms so as to match the load impedance. Transistor Q.sub.2 's collector current is still split between R.sub.6 and the load resistance so that only half of Q.sub.2 's collector current is available for the load. However, the transformer steps this up by a factor of two so that the load current is equal to Q.sub.2 's collector current. Transformers have the same drawback as ferrite couplers, However, in that they decrease the bandwidth of the amplifier. A transformer with a higher turn ratio, so as to further increase the efficiency of the amplifier, would decrease the bandwidth even more.